1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling a floating zone of a semiconductor rod and more particularly relates to an apparatus for producing semiconductor monocrystals by a floating zone method which controls the length of the floating zone and the diameter of a crystallizing crystal.
2. Description of the Prior Art
In the prior art shown in FIG. 6, a floating zone 20 (melting zone) is formed between a semiconductor rod 16 on a melting side (a polycrystalline rod as a raw material) and a semiconductor rod 18 on a crystallizing side (a monocrystalline rod as a product) by supplying a high-frequency current to an induction heating coil 12, and the floating zone 20 and the adjacent space thereof are monitored by an industrial television camera 30 so as to detect the position of a crystallization boundary 24 and the diameter D.sub.s of the crystallization boundary of a crystal from composite picture signals from the television camera, as well as an angle .alpha. between a tangent A at a point C in the periphery of the crystallizing boundary and the axis X of a semiconductor rod 14. In operation, the diameter of the crystallizing crystal has been controlled by adjusting the downward velocity V.sub.p of the melting-side semiconductor rod in accordance with the value of the angle .alpha..
The diameter D.sub.s at the crystallization boundary of the crystal is determined as a value which is proportional to a pulse width W.sub.1 of a pulse P.sub.1 of a luminance signal, as shown in FIG. 7, of a scanning line corresponding to the crystallization boundary 24. The angle .alpha. is determined from the difference between pulse widths W.sub.1 and W.sub.2 of pulses P.sub.1 and P.sub.2 of the luminance signals on this scanning line and a scanning line above it (refer to U.S. Pat. No. 3,757,071).
However, since it is necessary to determine the difference between the pulse width W.sub.1 and the pulse width W.sub.2 for adjacent scanning lines which are very close to each other, this small difference contains a large relative error factor and impairs the accuracy of detection of the angle .alpha.. In addition, since the error in detection of the position of the crystallizing boundary 24 is as large as 50% of the single interval of the scanning line, the accuracy of detection of the angle .alpha. is further deteriorated. It is also to be understood that, when there is a crystalline facet line near the point C, the detection accuracy is lowered even further.
Therefore, even if the diameter D.sub.s of the crystallization boundary of the crystal at a future time is predicted from the angle .alpha., the degraded prediction makes the ability to control the diameter D.sub.s of the crystallizing crystal poor.
In addition, even if the diameter of the growing crystal alone is controlled, there still are the following problems:
(1) As shown in FIG. 8, an unmelted cone 21 which is not yet molten and invisibly projects from the center of the bottom of the melting-side semiconductor rod 16 is present in the floating zone 20 that is formed by the induction heating coil 12 between the melting-side semiconductor rod 16 which moves downward and the crystallizing-side semiconductor rod 18. When a zone length L1 on the crystallizing side which is a distance between the induction heating coil 12 and the crystallizing boundary 24 is therefore decreased, and, if the lower end of the unmelted cone 21 is near the crystallization boundary 24, the temperature at the center of the crystallization boundary 24 becomes lower than the peripheral portion and the crystallization speed is thus locally increased, resulting in the occurrence of a dislocation or polycrystallization in an extreme due to the rise in the center of the boundary.
(2) When the cooling of the crystallization boundary 24 progresses further, the center thereof rises and the lower end of the unmelted cone 21 is brought into contact with the top of the crystallization boundary 24, the monocrystalline semiconductor rod 18 on the crystallizing side adhering to the polycrystalline semiconductor rod 16 on the melting side. Therefore, it is impossible to continue the floating zone method.
(3) Conversely, a zone length L which represents the axial length of the floating zone 20 becomes too long, the diameter of a constricted melt portion 35 is reduced, and the floating zone 20 is separated at the position of the constricted melt portion 35 by the surface tension, the melt consequently dropping.
(4) When gaseous impurities are injected into the floating zone 20 from the surface thereof (gas doping), or the impurities in the floating zone 20 are removed in a vacuum atmosphere (vacuum method), since the surface area of the floating zone 20 changes with the change in the zone length L, the doping or removal speed of the impurities changes and thus the resistivity of the semiconductor rod 18 on the crystallizing side becomes non-uniform in the axial direction.
On the other hand, if the electrical power supplied to the induction heating coil 12 is changed, the diameter D.sub.s at the crystallization boundary of the crystal and the zone length L also change. In addition, the diameter D.sub.s at the crystallization boundary of the crystal and the zone length L also are changed by a change in the downward velocity of the semiconductor rod 18 on the crystallizing side.
If the responsiveness of the control is poor or stable control cannot be achieved, the zone length becomes too short or too long, and thus the above-described crystal dislocation or adhesion occurs or the floating zone is cut. As a result, the quality of a product deteriorates or the floating zone method cannot be continued. In the cone portion in the initial state of crystal growth, even when there is no problem with respect, for example, to quality, the excessive zone length causes the cone length to be unnecessarily increased and results in product losses.
It is therefore highly desirable to provide a method of adjusting the amount of electrical power supplied to the induction heating coil 12 and the downward velocities of the crystallizing-side semiconductor rod 18 and the melting-side semiconductor rod 16 (and the relative velocity therebetween) so that the zone length L and the diameter D.sub.s at the crystallization boundary of the crystal can be stably controlled with good responsiveness.